Roofing granules, both natural and artificially colored granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are embedded in the asphalt coating on the surface of the asphalt-impregnated base material, the granules thus forming a coating that provides an adherent, weather-resistant exterior roofing surface. These shingles sometimes develop dark blotches or streaks on the surfaces of asphalt shingles, especially in warmer humid climates, because of the growth of algae and other microorganisms. Algae typically causes stains on shingles, which not only hurts aesthetics, but can also dislodge granules causing damage to shingles. In addition, algae holds water which can damage roofs, shortening roof life, and causes leaking
Some methods used to remedy such roof discoloration can dislodge protective granules such as with power washing or bleaching and can be followed with short-lived, periodic topical treatments using organic algaecides or inorganic metal salt solutions. Copper or zinc metal ridge flashing can be added as a pre- or post-installation step to the roof that can act as a longer term inhibitor. Zinc and copper flashing have been replaced over the years with aluminum and galvanized steel due to cost. Antimicrobial roofing systems have been used for many years and have employed various antimicrobial substances to deter naturally occurring growths on the roofing system. A widely used antimicrobial substance is copper. A common method currently in use provides special granules containing copper that are mixed with the colored granules on roof coverings during manufacturing, from about 1% to about 15% with about 8% to about 10% by weight the most common concentration.
The copper granules are typically produced by the manufacturers of color granules (for example, 3M and ISP Minerals) for use on roof coverings that use colored granules on the exposed surface of the roofing product. The copper granules are made up of a solid rock base material and coated with a copper containing coating that is kiln fired on the base rock. The copper granules usually come in black, white or gray colors. The copper granules are then mixed with other colored granules and applied to the roof covering product during the product manufacturing process. The copper-containing granules leach copper ions during the service life of the algae-resistant shingle to inhibit algae growth on the shingle surface even when the shingle surface is exposed to substantial moisture. The algae inhibitor has more localized coverage on the shingle as opposed to coming from a remote strip.
While algae-resistant shingles using copper-containing granules have been well known for many years, they are costly due to the additional cost of producing the copper-containing granules. Therefore, what is needed is an improved roofing product and method for providing resistance to naturally occurring growths on roofing systems that cause discoloration and/or deterioration of granule covered roof coverings including asphalt shingles.